As a secondary battery for a portable electronic apparatus, a non-aqueous secondary battery (particularly, a lithium secondary battery; hereinafter, it will be also referred to just the battery) has been put into practical use and has been widely prevalent. Further, in recent years, a lithium secondary battery is attracting people's attention not only as a small-sized one for a portable electronic apparatus but also as a large-capacity device for being mounted on a vehicle or for electric power storage. For this reason, there has been an increasing demand for safety, cost performance, lifetime and the like.
Generally, a layered transition metal oxide represented by LiCoO2 is used as an active material for cathode constituting a non-aqueous secondary battery. However, the layered transition metal oxide is liable to provoke oxygen elimination in a fully charged state at a comparatively low temperature around 150° C. Since this oxygen elimination reaction generates heat, oxygen is further eliminated. Therefore, a thermal bursting reaction where oxygen is continuously eliminated can be provoked. Therefore, in the non-aqueous secondary battery having the cathode active material, an accident such as heat generation or fire catching may happen.
Particularly for a non-aqueous secondary battery in a large size and a large capacity for being mounted on a vehicle or for electric power storage, high safety is demanded. Therefore, it has been expected to use lithium manganate (LiMn2O4) having a spinel structure, lithium iron phosphate (LiFePO4) having an olivine structure and the like that are stable in structure and do not release oxygen at an abnormal occasion as a cathode active material.
Further, as a result of prevalence of a non-aqueous secondary battery for being mounted on a vehicle, a big increase in the using amount of the cathode active material is presumed. Therefore, exhaustion of resources corresponding to the elements constituting the cathode active material is becoming a problem. It is particularly demanded to reduce the use of cobalt (Co) having a low degree of presence in the earth crust as a resource. For this reason, it has been expected to use lithium nickelate (LiNiO2) or a solid solution thereof (Li(Co1−xNix)O2), lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4) or the like as a cathode active material.
In view of enhancing the safety and of preventing the exhaustion of resources, LiFePO4 has been widely investigated. As a result of the investigations, LiFePO4 has been practically used as a cathode active material due to improvements in fine pulverization of particles comprising LiFePO4, in substitution of Fe and P with other elements, in coating of carbon on the particle surfaces, etc.
Here, there is a problem for LiFePO4 that its average electric potential is as low as 3.4 volts as compared with other cathode active materials. In view of the average electric potential, a cathode active substance having a high-potential olivine type structure such as LiMnPO4 has been also studied. However it has been known that intercalation and deintercalation of Li is difficult in LiMnPO4 since its conductivity is lower than that of LiFePO4 (refer to the Patent Document 1).
For this reason, there is a proposal for substituting a part of Mn with other element for the purpose of increasing the charge/discharge capacity by improving the charge/discharge characteristics (refer, for example, to the Patent Document 2). There is also a proposal for an active material represented by the formula AaMb(XY4)cZd (in the formula, A is an alkali metal; M is a transition metal; XY4 is PO4 or the like; and Z is OH or the like) (refer, for example, to the Patent Document 3).
Further, there is another proposal for an active material represented by the formula LiMP1−xAxO4 (in the formula, M is a transition metal; A is an element where an oxidation number is +4 or less; and 0<x<1) where the P site is substituted with an element A (refer, for example, to the Patent Document 4). There is still another proposal for an active material represented by the formula Aa+xMbP1−xSixO4 (in which A is Li, Na or K; and M is a metal) where the P site is substituted with Si (refer, for example, to the Patent Document 5).